Research in this proposal focuses on the molecular and cellular mechanisms underlying embryonic vascular development. Previous studies have demonstrated that a number of growth factor/signaling molecules play essential roles during formation of the original blood vessel network in the embryo. These include VEGF, angiopoietin, specific Notch pathway components and also certain ephrins. Using the frog and chick embryos as model systems, we have preliminary evidence showing that two additional growth factors are involved in regulation of embryonic vasculogenesis. First, we show that signaling by hedgehog family proteins is essential for vascular tube formation. Second, we have evidence suggesting that apelin signaling through its G-protein coupled receptor, APJ, is also important for initial patterning of the vascular system. Two of the specific aims of this proposal are designed to further explore the role of these signaling pathways in vascular development. Finally, we are investigating the origin of the original vascular endothelial precursor cells (angioblasts) in the embryo. We propose to identify possible regulators of angioblast formation, by determining which transcription factors are required for expression of the definitive angioblast marker VEGFR-2 (also called ilk-1 or KDR). The specific aims of the experiments in the proposal are as follows; (1). To further characterize the function of hedgehog signaling in vascular development, 2) To explore the role of apelin signaling during vascular development, (3). To identify DNA regulatory elements essential for VEGFR-2 expression in angioblasts and thereby to determine which transcription factors are required for defining the angioblast lineage. The long term goal of our research is to understand the molecular mechanisms underlying patterning and development of embryonic vascular tissues. Given the conservation of basic mechanisms underlying vertebrate development, it is extremely likely that results obtained using chick and frog embryos will be directly applicable to understanding formation of the embryonic vasculature in other organisms, including humans. This research has broad significance because the basic mechanisms underlying embryonic vascular development are likely to be reiterated during blood vessel formation associated with tumorogenesis, wound healing and vascular repair.